This invention relates to a drive method of a CCD color image sensor and a color image input apparatus.
Hitherto, an image sensor has been widely used as an image pickup device for outputting an electric signal responsive to light and shade of an optical image by executing photoelectric conversion. As a kind of image sensor, a CCD color image sensor is known. The CCD color image sensor includes photoelectric conversion elements and a shift register. The photoelectric conversion elements are provided with predetermined color filters of R (Red), G (Green), B (Blue), etc., and receive light passed through the filters and accumulate charges responsive to the light reception amounts. The charges generated in each photoelectric conversion element are transferred to the shift register as a shift gate is open, and the shift register outputs the charges to an output section by CCD.
By the way, the CCD color image sensor is installed in a color image input apparatus of an image scanner, a copier, etc., for example, as an image pickup device. The color image input apparatus scans and reads an optically transparent sheet of a negative film, a positive film, etc., on which an image is formed (which will be hereinafter referred to as transparent original) and a light reflection sheet of print paper, photographic paper, etc., on which an image is formed (which will be hereinafter referred to as reflection original) by the CCD color image sensor. At this time, to read a negative film, for example, the negative film has nature being the hardest to allow B (Blue) light of three prime colors to pass through and thus the accumulation time of signal charges is prolonged as for the photoelectric conversion element provided with the B filter.
An electronic shutter is publicly known as a technique of changing the charge accumulation time of the photoelectric conversion element as desired. The electronic shutter is a technique to discard unnecessary charges occurring in the photoelectric conversion elements outside the time period for accumulating charges (signal charges) to be taken out as an output signal. FIG. 14 shows a configuration example of a CCD color image sensor 200 adopted in a related art to implement the electronic shutter. The CCD color image sensor 200 is a linear image sensor and includes a plurality of rows of linear photoelectric conversion element groups 204 each consisting of a plurality of photoelectric conversion elements 202 provided with filters of the same color. The CCD color image sensor 200 is provided linearly with a shift gate 206, a shift register 208, a shutter gate 210, and a drain 212 along each row of the linear photoelectric conversion element groups 204.
When signal charges are accumulated in the CCD color image sensor 200, the shift gate 206 and the shutter gate 210 are shut. When signal charges are read from the CCD color image sensor 200, the shift gate 206 is opened and the signal charges are transferred to the shift register 208 after the signal charges are accumulated. In the time period until accumulation of the signal charges is again started after the signal charges are transferred to the shift register 208, the shift gate 206 is shut and on the other hand, the shutter gate 210 is opened and charges (unnecessary charges) occurring in the photoelectric conversion elements are discarded to the drain 212.
In the CCD color image sensor 200 in the related art, however, the photoelectric conversion element group 204 of each color is provided with the shutter gate 210 and the drain 212 for discarding the unnecessary charges and thus the structure becomes complicated and an increase in the manufacturing cost is incurred.